This invention relates to the field of ultrasound transducers and more particularly to ultrasound membrane transducers which comprise a membrane that is comprised of a diamond or diamond-like carbon layer.
The generation of ultrasonic sound takes place by purely mechanical means or by means of electroacoustic transducers which utilize the magnetostrictive or piezoelectric effect. Since ultrasound can be easily realized technically nowadays, it is widely used. Thus ultrasound is used for generating images in medical diagnostics or in non-destructive material testing.
The electroacoustical transducers used most widely are based on the piezoelectric effect. In practice, one-dimensional or two-dimensional array systems are mostly used in addition to single-transducer systems. Two-dimensional array systems are particularly interesting for the display of three-dimensional images.
The excitation of the piezoelectric elements in acoustic transducers takes place either in an AC field with a frequency of a few kHz up to several MHz or, in particular in image generation, by short oscillation bursts with a basic frequency of a few MHz and relative bandwidths of up to 100%. The excursion of the piezoelectric elements in the field direction generates a continuous or pulsatory ultrasound wave in the coupled medium such as, for example, water or biological tissue. The reflections changing in dependence on the tissue density and the throughput times changing with the path length are utilized for image generation in medical diagnostics.
In an array of ultrasound transducers each transducer may comprise a membrane on which a piezoelectric layer and on top of the piezoelectric layer a first and a second electrode are provided. The change in length of the piezoelectric element excites the membrane into oscillation.
An array of ultrasonic membrane transducers may be formed as so-called piezoelectric micromachined ultrasound transducers (PMUT). The array of piezoelectric ultrasound transducers is provided directly on silicon. Such an ultrasound transducer may comprise besides a silicon substrate a membrane on which a piezoelectric layer and on top of the piezoelectric layer a first and a second electrode are provided. The membrane can be obtained simply through etching away of the silicon so that an opening is created. The change in length of the piezoelectric element excites the membrane into oscillation. To manufacture an array of such ultrasound transducers, several openings are generated for the creation of several membranes on one silicon substrate.
Diamond is a preferred material for acoustic wave devices since it exhibits properties which are superior to conventional acoustic wave materials. Diamond possesses a combination of properties such as a low coefficient of thermal expansion, high mechanical hardness, large thermal conductivity, and high Young""s modulus, that are uniquely suited for acoustic applications.
Due to its above mentioned properties diamond is used as membrane material in ultrasound transducers.
A piezoelectric layer may be manufactured by depositing the piezoelectric material in a spray process, in a spin process, in a dip process, in a chemical vapor deposition process, in a sputter process or a laser ablation process. The deposition temperatures for all these processes lie between 500xc2x0 C.and 800xc2x0 C. depending on the composition of the piezoelectric material. In addition, all processes are carried out in the presence of oxygen in order to improve crystallization of the piezoelectric material.
These rigid process conditions may lead to partial decomposition of a membrane that is comprised of diamond and thus to significantly reduced adhesion of the piezoelectric layer on the surface of the membrane.
Another problem occurs due to the fact that the hydrophobility of the diamond surface differs depending on the manufacturing conditions of the membrane. This makes it more difficult to manufacture a piezoelectric layer with good and strong adhesion to a membrane that is comprised of diamond.
In addition, the structure and the morphology of the piezoelectric layer is influenced by a membrane that is comprised of diamond. It has been noted that often instead of a single phase piezoelectric layer a second phase containing pyrochlor is obtained when depositing a piezoelectric material on a diamond substrate.
It is accordingly an object of the present invention to provide an improved ultrasound transducer which comprises a membrane that is comprised of diamond or diamond-like carbon and a piezoelectric layer which shows good and strong adhesion to the other layers/components of the ultrasound transducer.
This object is achieved by means of an array of ultrasound transducers which each comprise a membrane that is comprised of diamond or diamond-like carbon, a barrier structure, a piezoelectric layer, and a first and a second electrode disposed on the same surface of the piezoelectric layer. In a preferred embodiment the barrier structure comprises at least one layer of an oxide selected from the group of TiO2, MgO, Al2O3, HfO2, ZrTiO4, LaAlO3 and any combination of these compounds.
The barrier structure prevents decomposition of the membrane that is comprised of diamond or diamond-like carbon during the deposition of the piezoelectric layer. In addition, the barrier structure ensures that a single phase piezoelectric layer is obtained when depositing the piezoelectric material on the barrier structure instead of directly onto the membrane. Another advantage is that adhesion between a piezoelectric layer and such a barrier structure is stronger than the adhesion between a piezoelectric layer and a membrane that is comprised of diamond or diamond-like carbon.
In accordance with a described embodiment, an array of ultrasound transducers is obtained in which the piezoelectric layer strongly adheres to the lower barrier structure, in which the piezoelectric layer shows a single phase and in which the membrane is not affected by the rigid process conditions when manufacturing the piezoelectric layer.
According to a preferred embodiment an array of ultrasound transducers may be comprised of micromachined ultrasound transducers.
The invention also relates to an ultrasound transducer which comprises a membrane that is comprised of diamond or diamond-like carbon, a barrier structure, a piezoelectric layer, and a first and a second electrode disposed on the same surface of the piezoelectric layer, wherein the barrier structure comprises at least one layer of an oxide selected from the group of TiO2, MgO, Al2O3, HfO2, ZrTiO4, LaAlO3 and any combination of these compounds.